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Synapsids: Before dinosaurs ruled the Earth

Before dinosaurs ruled the Earth, at the end of the Palaeozoic Era, the land was dominated by the synapsids. The synapsids (the amniote line that includes mammals) were a highly successful group which occupied most niches during the late Carboniferous and the Permian periods, but at the end of the Palaeozoic Era most families were extinguished by the Permian-Triassic mass extinction (around 252 million years ago) with only the mammalian line surviving to the present day. In this entry we’ll look at some of the more peculiar synapsid groups, which have led to the evolution of mammals like us.

CHARACTERISTICS AND EVOLUTIONARY TRENDS

The clade Synapsida includes mammals and all other amniotes more closely related to them than to reptiles. The synapsids were the first amniotes to diversify and appeared about 320 million years ago, at the middle of the Carboniferous period. These first synapsids were characterized by the presence of only one temporal fenestra behind each orbit through which the jaw muscles pass. Synapsida literally means “fused arches” referencing to the zygomatic arches (because in the past scientist believed that synapsids had evolved from diapsid reptiles and so their arches were thought to be “fused”).

Archaeothyris.svgDrawing of Archaeothyris’s skull, in which we can see some of the characteristics of the synapsids, like the temporal fenestrae and caniniform teeth. Drawing by Gretarsson.

Other characteristics that appeared through their evolution were:

  • Differentiation of differently-shaped (heterodont) teeth.
  • Lower jaw formed by fewer bones.
  • Acquisition of a more erect posture and an endothermic metabolism.

The first groups of more primitive or “reptile-like” synapsids are informally called pelycosaurs, while the latter more advanced forms are called therapsids (clade Therapsida, which in fact derived from pelycosaurs). As we will see, the evolution of synapsids is of the kind of “one group, which includes the next group, which includes the next group”.

Synapsid treeModified evolutionary tree of the amniotes by Kenneth D. Angielczyk (2009).

THE ORIGIN OF SYNAPSIDS, THE PELYCOSAURS

CotylorhynchusDB2Reconstruction of Cotylorhynchus, a caseasaurian that grew up to 3 metres long. Drawing by Dmitry Bogdanov.

The first synapsids had a sprawling limb posture, low slung bodies and were probably ectothermic. If we look at the skull morphology, the earliest groups of synapsids were the caseasaurians (clade Caseasauria) characterized by their small heads, an overdeveloped snout and their huge bodies (they were probably ectothermic and slow-moving creatures). Yet, if we look at the postcranial skeleton, the earliest synapsids were two groups called the varanopids and the ophiacodontids (Varanopidae and Ophiacodontidae families) which were similar to varanids (through convergent evolution), and while the former were quite small and agile creatures, the latter developed bigger forms with huge heads.

varanopid ophiacodontidDrawings of the varanopid Varanodon (top) and the ophiacodontid Ophiacodon (bottom). Both drawings by Dmitry Bogdanov.

Just before the appearance of the more advanced therapsids, the last two groups of “pelycosaurs” evolved and occupied most land ecosystems. Both groups shared a tall sail along their backs (similarly to Spinosaurus) formed by tall neural spines. In life, this sail probably was covered in skin and had plenty of blood vessels. Although it’s believed that these two groups were still ectothermic, this sail was probably used to gain or lose heat more easily.

Ianthasaurus_species_DB15_2Reconstruction of different species of edaphosaurids of the genus Ianthasaurus, showing their characteristic sail. Drawing by Dmitry Bogdanov.

The first of these groups is the Edaphosauridae family. Unlike most basal synapsids, the edaphosaurids were herbivorous and, along with the caseasaurians, they were among the first large amniotes to adopt a vegetarian lifestyle. The sails of edaphosaurids were covered with spiny tubercles, of which their function is still debated.

EdaphosaurusSkeleton of Edaphosaurus from the Field Museum of Chicago, where the tubercles on its spines are shown. Image by Andrew Y. Huang (2011).

The other group, the Sphenacodontidae family, were the sister group of the therapsids, inside the clade Sphenacodontia. While all other pelycosaur clades had their teeth loosely set in the jaw, the sphenacodontians had their teeth set in deep sockets. Most sphenacodontids were carnivorous, with strong jaws and well-developed caniniform teeth. Some species became the top predators on land before the apparition of the therapsids.

Dimetrodon_gigashomog_DBReconstruction of the sphenacodontid Dimetrodon, by Dmitry Bogdanov.

THE FIRST THERAPSIDS

Biarmosuchus_tener_skeleton_234Skeleton of Biarmosuchus, a basal therapsid in which we can see its more erect posture. Image by Ghedoghedo.

The therapsids (clade Therapsida, “beast arches”) appeared around 275 million years ago and replaced the pelycosaurs as the dominant land animals in the middle Permian. Early therapsids already had a more erect posture, unlike the sprawling limbs of the pelycosaurs. Also, their temporal fenestrae were larger, which made their jaws more powerful.

Estemmenosuchus_uralensisReconstruction of Estemmenosuchus, a dinocephalian from which fossil skin imprints have been found and so it’s known that it was covered in smooth, glandular skin without scales. Drawing by Mojcaj.

The therapsids diversified greatly and developed some extraordinary adaptations. The dinocephalians (clade Dinocephalia, “terrible head”) developed bony head knobs which are believed to be involved in some kind of head-butting behaviour. Another group, the anomodonts (clade Anomodontia, “abnormal teeth”) were characterized by having no teeth except for a pair of upper canines (which were probably covered by a beak). The anomodonts were the sister group of theriodontians.

Placerias1DBReconstruction of Placerias, an anomodont which could weigh up to one tonne. Drawing by Dmitry Bogdanov.

THERIODONTIA AND THE FIRST SABER-TEETH

Theriodontians (clade Theriodontia “beast teeth”) became the most successful group of synapsids. The three main groups probably looked pretty mammal-like, with fully-erect posture, a secondary bony palate which allowed them to breathe while swallowing or holding a prey and heterodont teeth (incisiviform, caniniform and molariform teeth). The most primitive theriodontian group were the gorgonopsians (clade Gorgonopsia, Gorgonopsidae family). All members of this group were carnivorous and active predators, as revealed by their sabre-toothed teeth. Although most of them were of a modest size, the larger ones reached up to 3 metres long and had canines of up to 15 cm long.

Inostrancevia_4DBReconstruction of Inostrancevia, the largest gorgonopsid genus, preying upon Scutosaurus, a parareptilian. Drawing by Dmitry Bogdanov.

A second group, the therocephalians (clade Therocephalia, “beast head”), were pretty more advanced than the gorgonopsians, although they didn’t reach their cousins’ size. Their feet resembled those of early mammals, they presented small pits on their bones which probably supported whiskers on fleshy lips, and most evidence suggests that they were already endotherms.

Pristeroognathus_DBReconstruction of a pair of Pristerognathus, a therocephalian genus in which we can see some more mammalian characteristics. Drawing by Dmitry Bogdanov.

Both gorgonopsians and therocephalians disappeared at the end of the Permian. The only therapsid group that survived through the Mesozoic period and that coexisted with the dinosaurs were the cynodonts

SMALL CYNODONTS

The cynodonts (clade Cynodontia “dog teeth”) appeared at the late Permian and diversified greatly along with the archosaurs. Although it is not really proven, most paleoartists represent cynodonts covered in fur, as evidence suggests an endothermic metabolism. Some characteristics of the cynodonts were:

  • Lower jaw formed only by the dentary bone, while the other jaw bones became the ossicles of the middle ear (the articular, the quadrate and the angular bones evolved into the malleus, the incus and the stapes).
  • Complex teeth: incisors to hold, canines to pierce, and premolars and molars to chew.
  • Only two sets of teeth (diphyodonts), instead of constantly-renewing teeth (polyphyodonts like most reptiles).
  • Large brain cavities. Some fossil burrows of different cynodonts have been found, revealing complex social behaviours.
thrinaxodon_by_ntamuraReconstruction of Thrinaxodon, a burrowing cynodont with whiskers and hair. Image by Nobu Tamura.

Even if they competed with archosaurs, some early forms became quite large. For example, some carnivores, like Cynognathus, had a large head and measured 1 metre long, while Trucidocynodon was about the size of a leopard. Yet, the evolutionary trend would make the cynodonts smaller, like the Brasiliodontidae family which, like most cynodonts, lived in the shadow of dinosaurs and other bigger reptiles. Brasiliodontids are thought to be the sister group of the Mammaliaformes (mammals and their most recent relatives).

Brasilitherium_riograndensisReconstruction of Brasilitherium, one of the most advanced non-mammalian cynodonts, which was only 12 cm in length. Drawing by Smokeybjb.

Finally, mammals appeared at the end of the Triassic period around 225 million years ago. The first mammaliaforms were probably, insectivorous, nocturnal shrew-like animals. It is thought that this nocturnal lifestyle is what actually propelled the development of fur coats, because in therapsids endothermy appeared before fur did. These mammaliaforms probably had mammary glands to feed their young when they had no teeth, but they probably had no nipples like current monotremes.

MegazostrodonLive reconstruction of Megazostrodon, a small mammaliaform which represents very well the transition from cynodonts to modern mammals. Image by Udo Schröter.

After the extinction of most archosaurs at the end of the Cretaceous period, the surviving synapsids took over the empty ecological niches. Mammals have ruled the world since then, conquering the land, the sea and even the air, but it wouldn’t have been possible without all the different adaptations acquired by early synapsids throughout their evolution. Thanks to them, humans and all other mammals are currently the dominant animals on the planet.

REFERENCES

The following sources have been used during the elaboration of this entry:

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Sinàpsids: Abans que els dinosaures dominessin la Terra

Abans de que els dinosaures dominessin la Terra, a finals del Paleozoic la terra estava ocupada pels sinàpsids. Els sinàpsids (la línia d’amniotes que inclou als mamífers) van ser un grup molt exitós que va ocupar la majoria de nínxols des del final del Carbonífer i fins al Pèrmic, però al final del Paleozoic la majoria de famílies es van extingir a causa de l’extinció masiva del Pèrmic-Triàssic (fa uns 252 milions d’anys) de la qual només els mamífers van sobreviure fins a l’actualitat. En aquesta entrada veurem alguns dels grups de sinàpsids més peculiars, els quals van portar a l’evolució dels mamífers actuals.

CARACTERÍSTIQUES I TENDÈNCIES EVOLUTIVES

El clade Synapsida inclou als mamífers i a tots els altres amniotes més emparentats amb aquests que amb els rèptils. Els sinàpsids van ser els primers amniotes en diversificar-se i van aparèixer fa uns 320 milions d’anys, a meitats del Carbonífer. Aquests primers sinàpsids es caracteritzaven per la presència d’una única fenestra temporal darrera de cada òrbita, a través de la qual passen els músculs mandibulars. Synapsida vol dir literalment “arcs fusionats” fent referencia als arcs zigomàtics (perquè abans els científics creien que els sinàpsids havien evolucionat de rèptils diàpsids i per tant es creia que els seus arcs s’havien “fusionat”).

Archaeothyris.svgDibuix d’un crani de Archaeothyris, en el que podem veure algunes de les característiques dels sinàpsids, com la fenestra temporal i dents caniniformes. Dibuix de Gretarsson.

Altres característiques que van aparèixer al llarg de la seva evolució són:

  • Diferenciació de dents amb formes diferents (heterodonts).
  • Mandíbula inferior formada per menys ossos.
  • Adquisició d’una postura més erecte i un metabolisme endoterm.

Els primers grups de sinàpsids més primitiu i “reptilians” s’anomenen informalment pelicosaures, mentres que les formes més avançades s’anomenen teràpsids (clade Therapsida que de fet, van evolucionar dels pelicosaures). Com veurem, l’evolució dels sinàpsids és del tipus de “un grup que inclou al següent grup el qual inclou al següent grup”.

Synapsid treeArbre evolutiu dels amniotes modificat de Kenneth D. Angielczyk (2009).

ORÍGEN DELS SINÀPSIDS, ELS PELICOSAURES

CotylorhynchusDB2Reconstrucció de Cotylorhynchus, un caseasaure que creixia fins als 3 metres de llarg. Dibuix de Dmitry Bogdanov.

Els primers sinàpsids presentaven potes que els sortien dels costats, cossos baixos que arrossegaven per terra i probablement eren ectoterms. Si ens fixem en la morfología cranial, els primers grups de sinàpsids van ser els caseasaures (clade Caseasauria) caracteritzats pels seus caps petits, un musell superdesenvolupat i els seus cossos enormes (probablement eren criatures lentes i ecotèrmiques). Tanmateix, si ens fixem en l’esquelet postcranial, els primers sinàpsids foren dos grups anomenats varanòpids i ofiacodòntids (famílies Varanopidae i Ophiacodontidae) els quals s’assemblaven als varans (evolució convergent) i, mentre que els primers eren animals força petits i àgils, els segons van desenvolupar formes més grans amb caps enormes.

varanopid ophiacodontidDibuixos del varanòpid Varanodon (superior) i l’ofiacodòntid Ophiacodon (inferior). Dibuixos de Dmitry Bogdanov.

Just abans de l’aparició dels teràpsids (més avançats), els dos últims grups de “pelicosaures” van evolucionar i van ocupar la majoria d’ecosistemes terrestres. Ambdós grups compartien una alta vela que els recorria l’esquena (semblant a la del Spinosaurus) formada per les espines neurals. Durant la vida, aquesta vela probablement estaria coberta de pell i tindria força vasos sanguinis. Tot i que es creu que aquests animals encara eren ectoterms, aquesta vela probablement s’utilitzava per guanyar o perdre calor més eficaçment.

Ianthasaurus_species_DB15_2Reconstrucció de diferents espècies d’edafosàurids del gènere Ianthasaurus, mostrant la seva vela característica. Dibuixos de Dmitry Bogdanov.

El primer d’aquests grups és la família Edaphosauridae. A diferència de la majoria de sinàpsids basals, els edafosàurids eren herbívors i juntament amb els caseasaures, foren dels primers grans amniotes en adoptar un estil de vida vegetarià. Les veles dels edafosàurids estaven cobertes de tubercles espinosos, la funció dels quals encara no està clara del tot.

EdaphosaurusEsquelet de Edaphosaurus del Field Museum de Chicago, on es poden veure els tubercles a les espines neurals. Imatge de Andrew Y. Huang (2011).

L’altre grup, la família Sphenacodontidae, era el grup germà dels teràpsids, dintre del clade Sphenacodontia. Mentre que la resta de clades de pelicosaures tenien les dents pobrament fixades a les mandíbules, els esfenacodonts tenien les dents profundament insertades. La majoria d’esfenacodòntids eren carnívors, amb fortes mandíbules i dents caniniformes ben desenvolupades. Algunes espècies van esdevenir els majors depredadors terrestres just abans de l’aparició dels teràpsids.

Dimetrodon_gigashomog_DBReconstrucció de l’esfenacodòntid Dimetrodon, de Dmitry Bogdanov.

ELS PRIMERS TERÀPSIDS

Biarmosuchus_tener_skeleton_234Esquelet de Biarmosuchus, un teràpsid basal en el qual podem veure la seva postura més erecte. Imatge de Ghedoghedo.

Els teràpsids (clade Therapsida, “arcs de bèsties”) van aparèixer fa uns 275 milions d’anys i van reemplaçar als pelicosaures com a animals terrestres dominants a mitjans del Pèrmic. Els primers teràpsids ja tenien una postura més erecte, a diferència de les extremitats laterals dels pelicosaures. A més, les seves fenestres temporals eren més grans, fent les seves mandíbules més potents.

Estemmenosuchus_uralensisReconstrucció de Estemmenosuchus, un dinocèfal del qual s’han trobat impresions fòssils de la pell, pel que se sap que estava cobert de pell llisa i glandular sense escames. Dibuix de Mojcaj.

Els teràpsids es van diversificar àmpliament i van desenvolupar algunes adaptacions extraordinàries. Els dinocèfals (clade Dinocephalia, “caps terribles”) van desenvolupar protuberàncies cefàliques òssies que es creu estaven relacionades amb algún tipus de comportament de competició entre mascles. Un altre grup, els anomodonts (clade Anomodontia, “dents anormals”), es caracteritzava per no tindre dents, excepte un parell de canins superiors (que probablement estaven coberts per un bec). Els anomodonts fóren el grup germà dels teriodonts.

Placerias1DBReconstrucció de Placerias, un anomodont que podia arribar a pesar fins a una tona. Dibuix de Dmitry Bogdanov.

THERIODONTIA I ELS PRIMERS DENTS DE SABRE

Els teriodonts (clade Theriodontia “dents de bèstia”) van esdevenir el grup de sinàpsids més exitós. Els tres grups principals probablement s’assemblaven força als mamífers, amb postures totalment erects, un paladar ossi secondari que els permetia respirar mentre empassaven o subjectaven una presa i dents heterodonts (dents incisiviformes, caniniformes i molariformes). El grup de teriodonts més primitius eren els gorgonopsis (clade Gorgonopsia, família Gorgonopsidae). Tots els membres d’aquest grup eren carnívors i depredadors actius, com revel·len les seves dents de sabre. Tot i que la majoria eren de mides modestes, els més grans arribaven als 3 metres de llarg i tenien canins de fins a 15 centímetres.

Inostrancevia_4DBReconstrucció de Inostrancevia, el gènere de gorgonòpsid més gran, depredant a Scutosaurus, un pararèptil. Dibuix de Dmitry Bogdanov.

Un segon grup, els terocèfals (clade Therocephalia “cap de bèstia”) eren força més avançats que els gorgonòpsids tot i que no van adquirir les mides dels seus cosins. Les seves potes s’assemblaven als dels primers mamífers, presentaven petits orificis als ossos que probablement aguantaven vibrises en llavis carnosos i tot apunta a que ja eren endoterms.

Pristeroognathus_DBReconstrucció d’un parell de Pristerognathus, un gènere de terocèfals en els quals podem veure algunes característiques més mamíferes. Dibuix de Dmitry Bogdanov.

Tant els gorgonòpsids com els terocèfals desapareixeren a finals del Pèrmic. L’únic grup de teràpsids que va sobreviure al llarg del Mesozoic i que coexistí amb els dinosaures van ser els cinodonts.

PETITS CINODONTS

Els cinodonts (clade Cynodontia “dents de gos”) van aparèixer a finals del Pèrmic i es van diversificar àmpliament juntament amb els arcosaures. Tot i que no està demostrat, la majoria de paleoartistes representen als cinodonts coberts de pèl, ja que les proves suggereixen un metabolisme endoterm. Algunes característiques dels cinodonts eren:

  • Mandíbula inferior formada únicament per l’òs dentari, mentre que els altres òssos mandibulars van esdevenir els ossets de l’oïda (l’articular, el quadrat i l’angular van evolucionar en el martell, l’enclusa i l’estrep).
  • Dents complexes: incissius per subjectar, canins per perforar, i premolars i molars per mastegar.
  • Només dos jocs de dents (difiodonts), enlloc de dents que es renoven constantment (polifiodonts, com la majoria de rèptils).
  • Grans cavitats cerebrals. S’han trobat alguns caus fòssils de diferents cinodonts, revel·lant comportaments socials complexes.
thrinaxodon_by_ntamura
Reconstrucció de Thrinaxodon, un cinodont excavador amb bigotis i pèl. Imatge de Nobu Tamura.

Tot i que competien amb els arcosaures, algunes formes primitives es van fer força grans. Per exemple, alguns carnívors com Cynognathus tenien caps grans i medien 1 metre de llarg, mentre Trucidocynodon tenia la mida d’un lleopard. Tanmateix, la tendència evolutiva faria que els cinodonts es féssin cada cop més petits, com la familia Brasiliodontidae la qual, com la majoria de cinodonts, va viure a l’ombra dels dinosaures i altres rèptils més grans. Els brasiliodontids es creu que foren el grup germà dels Mammaliaformes (els mamífers i els seus parents més recents).

Brasilitherium_riograndensisReconstrucció de Brasilitherium, un dels cinodonts no mamífers més avançats, que només media 12 cm de llarg. Dibuix de Smokeybjb.

Finalment, els mamífers van aparèixer a finals del període Triàsic, fa uns 225 milions d’anys. Els primers mamaliaformes probablement eren animals insectívors, nocturns i semblants a les musaranyes. Es creu que aquest estil de vida nocturn és el que va promoure l’aparició del pelatge, ja que en els teràpsids l’endotèrmia va aparèixer abans que el pèl. Aquests mamaliaformes probablement tinguéssin glàndules mamàries per alimentar a les seves críes quan aquestes no tenien dents, però probablement no tinguéssin mugrons com els monotremes actuals.

MegazostrodonReconstrucció de Megazostrodon, un petit mamaliaforme que representa força bé la transició de cinodonts a mamífers moderns. Imatge de Udo Schröter.

Després de l’extinció de la majoria d’arcosaures a finals del període Cretàcic, els sinàpsids supervivents van ocupar els nínxols ecològics buits. Els mamífers han dominat el món des de llavors, conquerint la terra, el mar i fins i tot l’aire, però això no hagués estat possible sense les diferents adaptacions adquirides pels primers sinàpsids al llarg de la seva evolució. Gràcies a ells, els humans i tots els altres mamífers som actualmente els animals dominants del planeta.

REFERÈNCIES

Durant l’elaboració d’aquesta entrada s’han consultat les següents fonts:

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Spinosaurus: the first aquatic dinosaur?

Recently, the BBC documentary series “Planet Dinosaur” has premiered on TVE2. In this series the latest paleontological discoveries concerning the biology of dinosaurs are explained. On my last entry we talked about the theropod dinosaurs, one of which is the spinosaur, one of the largest predators that have ever existed. On this entry I’m going to explain some of the facts that paleontology has revealed about the lifestyle of this creature.

TAXONOMY

The spinosaur (scientific name Spinosaurus aegyptiacus) belonged to the Spinosauridae family, a group of specialized theropods which appeared during the late Jurassic and became extinct about 93 million years ago during the late Cretaceous. This group was characterized by being relatively large theropods, with conic teeth and long snouts similar to crocodiles, and elongated neural spines through its back forming a sail-like structure (that’s where the name Spinosauridae comes from, meaning spine reptiles).

Spinosauridscale
Comparition of the different sizes of various spinosaurids by Scott Hartman. From right to left: Irritator challengeri, Baryonyx walkeri, Suchomimus tenerensis and Spinosaurus aegyptiacus.

Some of the more famous members on this family are, the Baryonyx from Europe, which had long curved claws on its hands to capture the fish it fed on, similar to its close relative the Suchomimus from northern Africa. Furthermore, there was the smaller Irritator of about 3 metres tall found in Brasil and finally, the Spinosaurus from northern Africa, which measuring between 12 and 18 metres long and wheighing between 7 and 20 tons, was one of the biggest predators to ever walk on land.

HABITAT AND DISTRIBUTION

The genus Spinosaurus was distributed in the zone of what is now the north of Africa. This genus lived during the Cretaceous, appearing about 112 million years ago and disappearing about 97 million years ago.

94_mya_Texas_Geology
Map of the World 94 million years ago by Joshua Doubek, during the middle Cretaceous period.

During that period, the northern part of Africa was a very humid zone with high temperatures and lots of wetlands. Spinosaurs probably lived in areas with large rivers and mangrove forests next to the sea, where tidal movements flooded its habitat during certain seasons of the year. This is in accordance with the vision that spinosaurids preferred wet semiaquatic habitats with plenty of great fish to prey upon.

Spinosaurus1DBa
Reconstruction from 2010 of Spinosaurus aegyptiacus by Dmitry Bogdanov.

Currently there are two possible spinosaur species. The most famous is Spinosaurus aegyptiacus from Egypt, the species of which we have more information. A possible second species is Spinosaurus maroccanus from Morocco, which some authors consider simply as a subpopulation of Spinosaurus aegyptiacus.

FUNCTION OF NEURAL SPINES

Spinosaurs were discovered in 1912 from a fossil which included its characteristic dorsal spines. These spines grow up to a length ten times that of the vertebra from which they emerged.

The scarcity of spinosaur fossils means that the function of the spines is still a mystery for science, although there are some hypothesis. One of these is that the spines formed a “sail” along the back of the animal which was highly irrigated and helped the animal’s thermoregulation, as such a big animal probably would have had problems losing heat. Therefore its sail would have helped the spinosaur to evade overheating, orienting it towards fresh winds to cool down.

Subadult_Spinosaurus
Reconstruction of the skeleton of a subadult spinosaur (Japan Museum, photo by Kabacchi).

Another hypothesis tells us that the spines held a hump-like structure similar to that of camels, which the animal would have used as a fat reserve system to store fat to withstand periods with little available feeding resources.

Lots of paleontologists think that both hypothesis could be correct and that the spinosaur used the sail both to regulate its body temperature and also to store fat to resist periods of low prey abundance. It is also possible that the sail made the spinosaur appear bigger than it actually was and that they used it during mating rituals similar to those of the modern peacock.

FEEDING STRATEGIES

The Spinosaurus‘s skull shows adaptions to a piscivorous diet. The snout is longer and slender than on other theropods. Aside from this, observing the snout of Spinosaurus it has been seen that it presents a series of little holes similar to those found on crocodiles. It is thought that these structures indicate the presence of pressure receptors which helped them detect the movement of their preys underwater.

Spinosaurus
Upper jaw of Spinosaurus from the Museo di Storio Naturale di Milano, where the holes which possibly contained the pressure receptors can be seen.

While the teeth of most carnivorous theropods where curved and serrated on their posterior part to tear flesh, spinosaur teeth were conic in shape and had no serration, more similar to those of crocodiles. These teeth were more useful for catching and holding fast and slippery prey and to prevent them from escaping (for example, a fish). Also, various Spinosaurus fossils have been found to have between their theeth scales and bones of large prehistoric fish which probably populated many rivers during the Cretaceous period.

new_mawsonia_by_hyrotrioskjan-d5qjb39
Reconstruction by Joschua Knüppe of two Mawsonia species, the rests of which have been found between the teeth of Spinosaurus.

Nevertheless, it is generally believed that the spinosaur was probably an opportunistic predator, feeding mainly on fish, also hunting small dinosaurs when it had the opportunity and stealing prey from smaller predators using its great size to intimidate them.

POSTURE AND LIFESTYLE

Spinosaurs have traditionally been represented as bipedal animals, as most similarly-sized theropods have. Eventhough most fossils are actually pretty incomplete, it is known that its forelegs were more developed than in most theropods, having long curved claws.

Traditionally it was thought that Spinosaurus hunted in a manner similar to a grey heron, roaming through zones of shallow water, sinking its long snout underwater to detect prey using the pressure receptors, and catching fish with its jaws. It then, probably used its front legs as hands to tear its prey to small pieces easy to swallow.

spinosaurus_by_hyrotrioskjan-d5ate1h
Reconstruction by Joschua Knüppe of Spinosaurus aegyptiacus in hunting posture.

At the end of 2014 a new Spinosaurus fossil was discovered which has changed the view we had on this animal. For the first time, scientists found a fossil which shows the structure of the hind legs of this dinosaur and they have observed a number of characteristics not found in any other theropod not even in other spinosaurids. This fossil shows that the hind legs of Spinosaurus were much more massive than those of other theropod dinosaurs, in which the bones are usually hollow to make them more agile (like present day birds). Also, in this fossil the hind legs are actually much shorter in relation to the size of the animal than in any other theropod, leading some scientists to think that Spinosaurus was actually a quadrupedal animal. This has made some paleontologist think that maybe the lifestyle of the spinosaurs was much more similar to that of a crocodile and that they spent much more time living in water than on land, making the Spinosaurus the first known aquatic dinosaur.

spinosaurs_aegyptiacus_2014_by_rodrigo_vega-d7zj8yn
Reconstruction by Rodrigo Vega of Spinosaurus based on the skeleton found in 2014.

Anyway, many paleontologists argue that the biology of a species cannot be based on a single fossil and advise caution when generalizing to the whole species (the fossil could belong to an adult and a juvenile that died together or could even come from an individual which had suffered some kind of embryonic malformation that kept its legs from developing normally). Paleontology is a science in which with every new discovery we can unravel the tree of life and the evolution of the different groups of living beings. With a little of luck, future discoveries will enable us to clarify the anatomy of Spinosaurus aegyptiacus and define the lifestyle of such a unique and extraordinary reptile.

REFERENCES

The following sources have been consulted in the elaboration of this entry:

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Spinosaurus: el primer dinosaure aquàtic?

Recentment s’ha estrenat a La 2 de TVE la sèrie documental de la BBC “Planeta Dinosaure” on s’expliquen els descobriments paleontològics més recents relacionats amb la biologia dels dinosaures. En la entrada anterior vam parlar dels dinosaures teròpodes entre els qual es troba l’espinosaure, un dels depredadors més grans que ha existit mai. En aquesta entrada explicaré algunes de les coses que la paleontologia ha revelat sobre l’estil de vida d’aquest animal.

TAXONOMIA

L’espinosaure (de nom científic Spinosaurus aegyptiacus) pertanyia a la família Spinosauridae, un grup de teròpodes especialitzats que va aparèixer a finals del Juràssic i desaparegué fa uns 93 milions d’anys a finals del Cretàcic. Aquest grup es caracteritzava per ser teròpodes relativament grans, amb dents còniques i morros allargats semblants als dels cocodrils i espines neurals allargades recorrent l’esquena formant una estructura en forma de “vela” (d’aquí ve el nom Spinosauridae”, rèptils amb espines).

Spinosauridscale
Comparació de la mida de diferents espinosàurids per Scott Hartman. De dreta a esquerra: Irritator challengeri, Baryonyx walkeri, Suchomimus tenerensisSpinosaurus aegyptiacus.

Els membres més coneguts d’aquesta família són el Baryonyx d’Europa, que tenia unes grans urpes corbades a les potes del davant per a capturar els peixos dels quals s’alimentava de forma semblant al seu parent proper, el Suchomimus del nord d’Àfrica. A més, tenim al petit Irritator d’uns 3 metres d’alt descobert al Brasil i finalment, a l’Spinosaurus del nord d’Àfrica, que mesurant entre 12 i 18 metres de llarg i pesant entre 7 i 20 tones, va ser un dels depredadors més grans que ha caminat mai per terra.

HÀBITAT I DISTRIBUCIÓ

El gènere Spinosaurus es trobava distribuït per la zona corresponent al que actualment és el nord d’Àfrica. Aquest gènere va viure durant el Cretàcic, apareixent fa uns 112 milions d’anys i extingint-se fa uns 97 milions d’anys.

94_mya_Texas_Geology
Mapa del món fet per Joshua Doubek de fa 94 milions d'anys, a mitjans del Cretàcic.

Durant aquest període, el nord d’Àfrica era una zona humida d’altes temperatures i zones pantanoses. Segurament els espinosaures vivien en àrees amb grans rius i manglars a prop de la costa, on les marees inundaven el seu hàbitat en diferents èpoques de l’any. Això concorda amb la visió de que els espinosàurids preferien zones humides amb abundància de grans peixos dels que alimentar-se.

Spinosaurus1DBa
Reconstrucció del 2010 de Spinosaurus aegyptiacus per Dmitry Bogdanov.

Actualment hi ha dues possibles espècies d’espinosaures. La més famosa és Spinosaurus aegyptiacus de Egipte, que és l’espècie de la qual més coses sabem. Una segona espècie possible és Spinosaurus maroccanus del Marroc, que alguns consideren simplement una subpoblació de Spinosaurus aegyptiacus.

FUNCIÓ DE LES ESPINES DORSALS

Els espinosaures van ser descoberts el 1912 per un fòssil que incloïa les característiques espines dorsals. Aquestes espines arribaven a fer 10 vegades la longitud de les vèrtebres de les quals emergien.

La escassetat de fòssils d’espinosaures ha provocat que la funció de les espines sigui encara un misteri per la ciència. Tot i així, existeixen vàries hipòtesis. Una d’aquestes és que les espines formaven una “vela” al llarg de l’esquena altament irrigada amb vasos sanguinis que ajudava a l’animal en la termoregulació, ja que probablement un animal tant gran tingués problemes per a perdre calor. La vela hauria ajudat a l’animal a evitar el sobreescalfament orientat-la cap a la direcció del vent per a refrescar-se.

Subadult_Spinosaurus
Reconstrucció d'esquelet d'espinosaure subadult (Museu de Japó, foto de Kabacchi).

Una altra hipòtesi diu que les espines formaven una estructura més semblant a la gepa d’un camell que l’animal utilitzava per a emmagatzemar greix per a suportar períodes d’escassetat de recursos.

Molts paleontòlegs creuen que ambdues hipòtesis podrien ser correctes i que l’espinosaure utilitzés la vela tant per a regular la seva temperatura corporal i a més que també hi emmagatzemés greix per a resistir períodes de falta d’aliment. A més, la vela segurament feia que l’espinosaure semblés encara més gran del que era i també que la fessin servir en els rituals d’aparellament de forma semblant als paons actuals.

ALIMENTACIÓ

El crani de Spinosaurus mostra adaptacions a una dieta piscívora. El musell és més allargat i prim que en altres teròpodes. Apart d’això, en observar el morro del Spinosaurus s’ha observat que presenten un seguit de petits forats semblants als que presenten els cocodrils. Es creu que aquestes estructures indiquen la presència de receptors de pressió que els permetia detectar el moviment de les seves preses a sota de l’aigua.

Spinosaurus
Mandíbula superior de Spinosaurus del Museo di Storio Naturale di Milano, on s'observen els forats que possiblement contenien els receptors de pressió.

Mentre que les dents de la majoria de teròpodes carnívors eren corbades i presentaven una serra a la part posterior per a tallar la carn, les dents de l’espinosaure eren còniques i sense serra, més semblants a les d’un cocodril. Aquestes dents eren més útils per atrapar i impedir que preses ràpides i relliscoses se’ls escapessin (com per exemple un peix). A més, en varis fòssils de Spinosaurus s’han trobat entre les seves dents restes d’escates i ossos de grans peixos prehistòrics que segurament poblaven els rius del Cretàcic.

new_mawsonia_by_hyrotrioskjan-d5qjb39
Reconstrucció feta per Joschua Knüppe de dues espècies de Mawsonia, uns peixos dels quals s'han trobat restes entre les dents de Spinosaurus.

Tanmateix es creu que l’espinosaure devia ser un depredador oportunista, alimentant-se principalment de peixos, però caçant petits dinosaures quan podia o robant les preses a altres depredadors aprofitant-se de la seva mida.

POSTURA I ESTIL DE VIDA

Els espinosaures tradicionalment se’ls ha representat com a animals bípedes, igual que la majoria de teròpodes de mida semblant. Tot i que la majoria de fòssils trobats són bastant incomplets, es sap que les potes del davant estaven més desenvolupades que les de la majoria de teròpodes i que presentaven unes urpes llargues i corbades.

Tradicionalment es creia que Spinosaurus caçava d’una forma semblant al bernat pescaire, rondant per zones d’aigües poc profundes, enfonsant el seu llarg morro per a detectar a les preses mitjançant els receptors de pressió i capturant-la amb les mandíbules. Segurament, també utilitzava les potes del davant per a acabar de desmembrar les preses en troços més petits i menjar-se-les.

spinosaurus_by_hyrotrioskjan-d5ate1h
Reconstrucció feta per Joschua Knüppe de Spinosaurus aegyptiacus en postura de caça.

A finals del 2014 es va descobrir un nou fòssil de Spinosaurus que ha canviat la visió que es tenia sobre aquest animal. Per primer cop s’ha trobat un fòssil que mostra les potes del darrera d’aquest dinosaure i s’han vist un seguit de característiques que no es troben en cap altre teròpode i ni tan sols en cap altre espinosàurid. Aquest fòssil ens mostra que les potes del darrere eren molt més massisses que en altres dinosaures teròpodes, els quals solen tenir els ossos buits fent-los més lleugers (com en els ocells actuals). A més, en aquest fòssil les potes del darrere són molt més curtes en relació a la mida de l’animal que en cap altre teròpode, cosa que ens porta a pensar que Spinosaurus probablement fós un animal quadrúpede. Això ha provocat que alguns científics creguin que l’estil de vida d’aquests dinosaures era més semblant al del cocodril i que aquests passarien molt més temps vivint a l’aigua que a terra, convertint a l’espinosaure en el primer dinosaure aquàtic conegut.

spinosaurs_aegyptiacus_2014_by_rodrigo_vega-d7zj8yn
Reconstrucció feta per Rodrigo Vega de Spinosaurus basant-se en el esquelet descobert el 2014.

De tota manera, molts paleontòlegs argumenten que no es pot basar la biologia d’una espècie en un únic fòssil i aconsellen precaució a l’hora de fer generalitzacions a tota l’espècie (el fòssil podria tractar-se d’una barreja d’ossos d’individus adults i juvenils o fins i tot podria ser un individu que va patir una mutació congènita que fes que les potes posteriors no es desenvolupessin correctament). La paleontologia és una ciència que amb cada nou descobriment va desentranyant l’arbre de la vida i la evolució dels diferents grups d’éssers vius. Amb una mica de sort, descobriments posteriors ens permetran aclarir l’anatomia del Spinosaurus aegyptiacus per així poder acabar definint l’estil de vida d’aquest rèptil tant únic i extraordinari.

REFERÈNCIES

S’han consultat les següents fonts per a elaborar els continguts d’aquesta entrada:

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The kingdom of the reptiles: what is a dinosaur?

Dinosaurs (superorder Dinosauria, “terrible reptiles”) are a group of reptiles which dominated all terrestrial ecosystems during the Mesozoic (Secondary Era or “the Age of Reptiles”). Even today, to most people there’s still some confusion over what a dinosaur is and what is not, and the term “dinosaur” is often used to refer to all the reptiles that evolved during the Secondary Era. In this entry I’ll try to give account of some of the different groups of reptiles that appeared during the Mesozoic and I’ll explain the classification of the different dinosaurian groups and some of their adaptations.

MESOZOIC REPTILES: DINOSAURS AND LOTS MORE

The rise of the dinosaurs was possible thanks to a mass extinction phenomenon which occurred 251 million years ago (Permian-Triassic extinction event). That phenomenon annihilated up to 96% of marine species and up to 70% of terrestrial species in that time, leaving lots of empty ecological niches to be inherited by new animal species.

Sin título
Modified graphic from Rohde & Muller (2005) showing the great massive extinction. The darker zone corresponds to the Mesozoic period.

During the Triassic period (in the early Mesozoic) many different groups of reptiles evolved. One of these groups was the Dinosauria, which at that moment was far from being the dominant group of terrestrial animals. Some other reptilian groups of that time were the terrestrial rauisuchians (clade Rauisuchia) and fully aquatic groups like the sauropterygians (superorder Sauropterygia) and the ichthyopterygians (superorder Ichthyopterygia).

Reconstructions by Dmitry Bogdanov of Prestosuchus (a rauisuchian, top), Nichollsia (a suropterygian, left bottom) and Platypterigius (an ichthyopterygian, right bottom).
Reconstructions by Dmitry Bogdanov of Prestosuchus (a rauisuchian, top), Nichollsia (a suropterygian, left bottom) and Platypterigius (an ichthyopterygian, right bottom).

A second mass extinction in the late Triassic and the early Jurassic put an end to most of the dominant reptile groups, allowing the yet small dinosaurs to expand and evolve, along with some new groups like the crocodilomorphs (superorder Crocodylomorpha, ancestors of crocodilians), the flying pterosaurs (order Pterosauria).

Reconstructions by Dmitry Bogdanov of Dakosaurus (a crocodilomorph, top) and Scaphognathus (a pterosauria, bottom).
Reconstructions by Dmitry Bogdanov of Dakosaurus (a crocodilomorph, top) and Scaphognathus (a pterosauria, bottom).

As we can see, dinosaurs are only one of many reptile groups that evolved during the Mesozoic. During the Jurassic period, dinosaurs diversified into many different groups, but they were mostly restricted to terrestrial ecosystems, which they would rule until their practical extinction 65 million years ago at the end of the Cretacic period.

DINOSAUR CLASSIFICATION

The first dinosaurs evolved around 231 million years ago during the mid-Triassic period. They were small in size and were characterized by their limb’s posture, which contrary to most reptiles, grew vertically elevating their body from the ground. That gave them more agility and a more active lifestyle.

eorpohd
Top: Skeleton of Eoraptor, one of the oldest known dinosaurs (Museum of Japan, photo by Kentaro Ohno). Bottom: Representation of the posture common among most reptiles (left) and the posture characteristic of dinosaurs (right).

Then dinosaurs diverged into two different orders: the Saurischia and the Ornithischia. These two groups were distinguished by the structure of their pelvis; saurischians conserved a pelvis more closely similar to that of the other reptiles, while the ornithischians evolved a pelvis superficially similar to that of modern birds.

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Representation showing the structure of saurischian hips (left) and ornitischian hips (right). The animals represented are facing left.

ORNITISCHIA: BIRD HIPPED

ORNITHISCHIA
Evolutionary tree of Ornithischia, modified by Zureks.

Ornithopoda (“bird feet”): Ornithopods were the most diverse group of Ornithischia, characterized by their three-toed feet similar to that of birds. They were herbivores that could combine bipedal and quadruped walking. Among them we can find the Iguanodon, one of the first dinosaurs to be discovered by science.

IGUANODOOOONT
Iguanodon feet (right) and reconstruction by O. C. Marsh (1896).

Ornithopods acquired many different adaptations; some groups had duck-like bills to feed on aquatic vegetation, others developed specialized hands with a sharp thumb and an opposable little finger to grasp the plants they fed on. Many groups developed bony crests which are thought to be used both for species identification and for communication between members of the same species.

parasauro
Reconstruction of Parasaurolophus (missing author), an ornithopod which presented a big hollow crest to amplify the sounds it made.

Marginocephalia (“fringed heads”): The so-called marginocephalians were a group of herbivorous dinosaurs related to the ornithopods characterized by a great cranial ossification. These can be divided into two separated groups:

Pachycephalosaurians (suborder Pachycephalosauria, “thick-headed reptiles”) were bipeds which had an extremely thick skull and a series of lateral osteoderms (keratin-covered ossifications) flanking it. It is believed that pachycephalosaurians resolved territorial fights and disputed reproductive rights via head-ramming, similar to goats.

Pachycephalosauria_jmallon
Reconstruction of Pachycephalosaurus by Jordan Mallon.

The other members of the group are the ceratopsians (suborder Ceratopsia, “horned faces”), quadrupeds which presented; neck frills making their skulls look bigger and the “rostral bone”, which formed a beak-shaped structure on the mouth. Lots of species also developed facial horns which could protrude from the cheek-bone, the eyebrow or the neck frill.

CERAAA
Reconstruction of Rubeosaurus by Lukas Panzarin (left) and skull of Triceratops (right), photo by Zachi Evenor.

Thyreophora (“shield bearers”): This basal group of ornithischians was exclusively composed of quadruped herbivores characterized by the presence of heavy osteoderms that constituted their main defence. This group can be divided into:

Stegosaurians (suborder Stegosauria, “roofed reptiles”) were big herbivorous dinosaurs characterized by having two rows of dorsal osteoderms from the neck to the tail, which served as protection and helped them in their thermoregulation. Some species also developed caudal spines called “thagomizers” used as weapons to defend themselves from predators.

STEGo
Mounted “thagomizer” at Denver Museum of Nature and Science (left) and reconstruction of Stegosaurus by Nobu Tamura (right).

Anchylosaurians (suborder Ankylosauria, “fused reptiles”) developed heavy bony armours that covered most of the body. Some of them, like the Ankylosaurus, developed big bony clubs at the end of the tail to fend off predators.

Euoplocephalus_BW
Reconstruction of Euoplocephalus by Nobu Tamura.

SAURISCHIA: REPTILE HIPPED

SAURISCHIA
Evolutionary tree of Saurischia, modified by Zureks.

Sauropodomorpha (“reptile-shaped feet”): The sauropodomorphs are better known as the “long-necked dinosaurs”. That’s because they adapted to feed on the highest strata of vegetation.

Macronaria_scrubbed_enh
Reconstruction of different sauropodomorphs (left to right): Camarasaurus, Brachiosaurus, Giraffatitan and Euhelopus.

Most species became large quadrupeds, with pillar-like legs similar to those of elephants and long necks to reach the leaves of the highest trees. Later species reached tremendous sizes, like the Amphicoelias which could grow up to 60 metres long.

Theropoda (“beast feet”): This last group is mostly known for two reasons. First of all is that this group includes some taxons of great predators like the Tyrannosauridae and Dromeosauridae families. The second reason is that theropods are the only dinosaurian group that includes living species, because modern birds are included in the suborder Theropoda.

AllosaurusAMNH5753
Skeleton of Allosaurus from american museum collections (1915).

All theropods are bipedal and most of the Mesozoic species were carnivorous, with sharp replaceable teeth adapted to predation. Theropods present a saurischian pelvis but later on, birds evolved a hip structure more similar to that found in ornitischian dinosaurs.

Davide-Bonnadonna-theropod-size-evolution-600-px-tiny-July-August-Darren-Naish-Tetrapod-Zoology
Reconstruction by Davide Bonnadonna of the different clades that led to the aparition of birds (left to right): Neotheropoda, Tetanurae, Coelurosauria, Paraves and finally the Archaeopteryx, believed to be the first bird species that ever existed.

Some species had feathers to help thermoregulation. Birds from these groups evolved at the end of the Jurassic period.

REFERENCES

The following sources have been consulted in the elaboration of this entry:

If you enjoyed this article, please share it on social networks tospread it. The aim of the blog, after all, is to spread science and reachas many people as possible.

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